This invention relates to a heat transfer sheet, and more particularly to a heat transfer sheet capable of producing an image which is excellent in color density, sharpness, and fastness, in particular, preservability.
Heretofore, a variety of heat transfer printing methods have been proposed. Of these, a sublimation-type heat transfer printing method is now prevailing, in which a heat transfer sheet comprising a sublimable dye as a coloring agent (printing agent) which is retained by a substrate sheet such as paper is superposed on a heat transfer image-receiving sheet such as woven cloth of polyester fiber which is receptive to the sublimable dye, and thermal energy is then applied imagewise to the back surface of the heat transfer sheet, whereby the sublimable dye is transferred to the heat transfer image-receiving sheet to produce an image therein.
Recently, a heat transfer printing method of the sublimation-type has been proposed which can produce a full-colored image on an image-receiving sheet such as a sheet of paper or a plastic film. In this method, a thermal head of a printer is employed as a heat application means, and a large number of dots in three or four colors are transferred to the image-receiving sheet in an extremely short heat application time. A full-colored image can thus be successfully reproduced on the image-receiving sheet.
The image thus obtained is very sharp and clear because a dye is used as a coloring agent. Therefore, the heat transfer printing method of this type can produce an excellent half-tone image with continuous gradation, comparable to an image obtained by offset printing or gravure printing. Further, the quality of the image is as high as that of a full-colored photograph.
However, an image produced even by the above printing method is still suffering from the problems of insufficient color density, low preservability, and discoloration which tends to be caused during the preservation thereof over a long period of time.
In order to conduct a high-speed printing, it is required that thermal energy be applied to the heat transfer sheet in an extremely short time of several seconds or less. However, both the sublimable dye contained in the heat transfer sheet, and the heat transfer image-receiving sheet are not sufficiently heated during such a short heat application time. A resulting image, therefore, cannot have sufficiently high color density.
A sublimable dye having high sublimation ability has been developed in order to successfully achieve high-speed heat transfer printing. In general, however, a highly sublimable dye has a low molecular weight. Therefore, when such a dye is employed in a heat transfer sheet, and is transferred to an image-receiving sheet, it tends to easily migrate in the image-receiving sheet, or to bleed out the surface thereof with the passage of time. For this reason, the image produced by the highly sublimable dye has low preservability; more specifically, the image is blurred or its sharpness is reduced during the preservation thereof. In addition, the bled dye stains an article which is brought into contact with the image-receiving sheet.
In order to eliminate the above problems, it may be considered to employ a sublimable dye having a relatively high molecular weight. Such a sublimable dye, however, cannot sublime instantly upon application of heat, so that an image having high color density cannot be obtained by high-speed printing.